The invention relates to a separation structure for selecting the propagation mode of the ligh waves in a light wave guide, the latter being constituted by a strip of a semiconductor material having a first refractive index formed on a semiconductor substrate of a material having a second refractive index lower than the first index and such that the light is confined in the strip forming the guide, this structure being arranged in the path of the light beam in such a manner that it de-limits in the guide at least one region in which the propagation of the light wave is monomode and a second region in which the propagation is bimode.
The invention further relates to an optical switching element between two light wave guides including such structures and to an optical switching matrix constituted by these elements.
The invention is used in the field of switching optical signals, transported through optical fibres, for example, in the field of telecommunication, in which the switching between numerous optical fibres has to be made possible in order to avoid having to convert the optical signals transported through fibres into electric signals during switching between the different subscribers.
An optical switching matrix constituted by switching elements comprising such separation structures is known from the publication of A. Neyer and W. Mevenkamp entitled "Single-mode electrooptic X-switch for integrated optical switching networks" in I.E.E.E. Second European Conference on Integrated Optics (Fiorensa, 17-18 Oct. 1983, Conference Publication No. 227, p. 136-139).
This document describes a switching matrix of nxn light wave guides monolithically integrated on a substrate of LiNbO.sub.3. Monomode light wave guides are formed in the substrate by diffusion of Ti. These guides have a width w and cross each other pairwise at an angle .alpha..
The intersection zone of two guides constitutes the switching element. The variation of the refractive index with respect to the substrate y is two times that of a single guide. This zone has a dimension d parallel to the substrate that is larger than the dimension w on a single guide. Therefore, the intersection zone of two guides is bimode in a plane parallel to the substrate. It transports both the fundamental mode and the transversal mode of the first order.
This device permits switching between the two output guides. The power transported by each of the output guides is determined by the relative phase difference betwen the two fundamental first order modes at the end of the intersection zone. This phase difference depends upon the width w, upon the variation of the refractive index and upon the angle .alpha. to the intersection.
This phase difference can be electrically controlled by an electrooptical variation of the refractive index by polarizing two electrodes arranged on either side of the intersection zone of each assembly of two guides.
However, this known device has several disadvantages. In the first place, the intersection angles .alpha. of the two guides are small, i.e. from 0.6.degree. to 1.4.degree.. Consequently, they can be formed only with difficulty on a semiconductor material. In fact, all the manufacturing methods that can be used to form the light wave guides and more particularly etching or epitaxial growth are anisotropic. Thus, all the operations not carried out parallel to crystallographic axes or surfaces give rise to roughness or steps in the walls of the guides and hence to losses. Angles of the order of 1.degree. cannot lead to crystallographic directions in the semiconductor material. The known device can therefore be obtained only with difficulty and is characterized by high losses. Furthermore, such angles necessarily lead to a large surface area for the matrix. In fact, for a matrix of 10.times.10 switching elements, the surface area is of the order of 20.times.20 mm. For integrated circuits, this surface area is really enormous.
Therefore, the present invention suggests a device which overcomes these disadvantages.